They report their evidence of a watery world in today's issue of the journal Science.

"If you were to have come by in a flying saucer 4.4 billion years ago and look down on a good day there would have been about as much continent as there is now," says Harrison.

"There would have been about as much water, it would have been blue, there would have been lots of volcanoes but it would have looked like the planet that we know and love."

This is contrary to what scientists have generally believed and suggests Earth might have been hospitable enough to nurture life from virtually the minute it came off the planetary production line.

The Hadean period got its name from the Hades underworld of Ancient Greek mythology because it was assumed to be a time of extreme turbulence and catastrophic events.

"This cartoon like view that we've had of the first 500 million years of Earth's history was based on no observational evidence whatsoever," Harrison says. "We made a myth about the earliest Earth."

A watery world

The idea that oceans had already formed in the Hadean got its first boost in 2001 when Harrison and colleagues published a study in the journal Nature on rare, ancient crystals called zircons.

Zircons are formed as magma cools slowly into granite beneath the Earth's surface and are the only geological materials surviving from the Hadean.

The researchers measured oxygen levels in zircon crystals found in rocks at Jack Hills, 350 kilometres north of Perth in Western Australia.

They found zircons as old as 4.4 billion years containing high levels of a particular isotope of oxygen that could only have come from clay.

Since clay can only form in water at the Earth's surface, they concluded that oceans were around at the time.

In the same issue of Nature, a group led by Professor Simon Wilde of Curtin University in Perth, working quite independently on zircons, reached the same conclusion.

This marked the birth of the so-called Hadean waterworld hypothesis.

New evidence

Watson and Harrison have now found that the 4.4 billion-year-old zircons from Jack Hills were formed at a much lower temperature than thought.

While it was previously believed that Hadean geology was violent because of regular meteorite impacts, Harrison says the new evidence supports the idea that by 4.4 billion years Earth had organised itself into a more gentle process of continent formation and recycling that is required for sustaining life.

"Life could have begun virtually on day one," says Harrison. "There was no turbulent adolescence."

Watson and Harrison have developed a special 'thermometer' that can measure the amount of titanium in zircons. The higher the titanium levels, the higher the temperature of the magma when the zircons crystallised.

The amount of titanium in the zircons suggest they were formed at a much lower temperature than previously suggested, a temperature of around 650 to 660°C, 10 kilometres below the Earth's surface.

If Earth was being constantly bombarded from space then the zircons would have been formed at much higher temperatures of around 900 or 1100°C, says Harrison.

He says the only way the zircons could have been made at this relatively low temperature is if the cooling magma was saturated with water.

"If we're right then we are essentially radically re-interpreting the earliest history of the Earth," he says.